Cell replacement therapy is a viable option as a long-term treatment for Huntington's disease (HD), where expansion of CAG repeats in Huntingtin (HTT) gene results in the degeneration of the majority of striatal neurons. The damaged neurons may be replaced by new functional neurons, which can be derived from induced pluripotent stem cells (iPSCs). However, iPSCs from the patients' cells harbor the same mutant HTT (mHTT) gene, which eventually will lead to toxic HTT protein accumulation and cell degeneration. Therefore, genetic repair should be done to reduce overproduction of mHTT proteins before cell replacement. Moreover, the integration of the replaced cells into the neuronal network is important for reconstituting neural functions. This study is evolved based on our recent success in developing a transgenic HD monkey model. HD monkeys develop cellular changes and neuropathologies similar to that of human patients, which are rarely observed in rodent models. Besides the neurotoxicity of mHTT in monkeys, HD monkeys also develop involuntary movement, difficulties in coordinating body movement, cognitive decline and striatal atrophy similar to that of HD patients. Our preliminary results show that iPSCs from our HD monkey can be in vitro differentiated into neural progenitor cells (NPCs) and GABAergic neurons, which form the majority of striatal neurons. We also show that NPCs grafted into severe combined immunodeficient (SCID) mice continue to differentiate into GABA expressing neurons after implantation and do not form tumors. These results suggest that NPCs derived from HD monkey/patients themselves may be utilized to replenish the population of the lost striatal neurons. However, it is unclear whether these NPCs will be able to restore the function of the striatum of HD. The overall objective of this proposal is to determine the effectiveness of NPCs replacement in rescuing the abnormal phenotype of HD mouse model. The proposed study will be used as a proof-of-principle which will lay the groundwork for future cell replacement therapy in HD monkeys by genetically correcting iPSC-derived NPCs from HD monkeys (rHD-NPCs). To achieve our goals, our approach is to suppress mHTT expression in rHD-NPCs with small hairpin RNA targeting the HTT gene (shRNA-htt) and then grafts the resulted rHD-NPCs into HD mice. Genetic modulation with shRNA can reduce the expression and subsequent accumulation of toxic mHTT, whereas cell replacement may improve the abnormal behavior and the atrophy of the striatum in HD mice model. To achieve our goals, we have two specific aims: (1) Determine neuronal differentiation properties of genetically modulated rHD-NPCs and (2) Determine the efficacy of rHD-NPC-siHD2 graft in rescuing the abnormal phenotype of HD mice.